A carbon fiber reinforced carbon composite material (hereinafter referred to as a C/C composite) is a material resulting from a great improvement in mechanical characteristic disadvantages, such as low toughness and brittle fracture, of artificial graphite materials that are used for electrodes or isotropic graphites. However, the C/C composite is a carbon material, too, and therefore disadvantageously suffers from oxidation loss in the air when exposed to a high temperature of 400 to 500 degrees C. or higher. Thus, the C/C composite is also limited in its scope of use. As solutions therefor, known are approaches of: (1) coating the C/C composite with ceramics excellent in oxidation resistance such as SiO2, B2O3—SiC, B4C, etc.; (2) conversion into SiC, etc.; (3) combination of (1) and (2) to give the C/C composite a functionally gradient characteristic; (4) impregnating the C/C composite with metal alkoxide to incorporate SiC, B4C, or oxides thereof into a whole of the C/C composite; (5) impregnating the C/C composite with B2O3 (Japanese Patent No. 3135129); (6) mixing B4C into a felt C/C composite during an ordinary production process therefor (Japanese Patent Laid-Open No. H05-306180), and the like.
However, the aforementioned approaches involve the respective problems that: (1) it is hard to maintain the oxidation resistance when a coating layer is damaged; (2) fibers become brittle, mechanical characteristics deteriorate, and vulnerability to thermal shock is increased; (3) costs become higher (this approach is unsuitable for general use although successful for aerospace applications); (4) the metal alkoxide is expensive; (5) a heating and impregnating device (device for applying hot isostatic press) is required and thus costs increase, and, in addition, the device is limited in size; (6) there can be obtained only a felt C/C composite of felt type that adopts a felt as a carbon fiber and a pyrolytic carbon as a matrix.
Japanese Patent No. 3058180 discloses a method of applying a mixture of boron carbide with a thermosetting resin, coal tar, or pitch to serve as a matrix, and a method of incorporating B4C into a C/C composite by means of thermal decomposition of boron-containing gas, to reveal that, at 1200 degrees C., the obtained C/C composite exhibits oxidation loss at a lower speed as compared with materials containing no boron carbide. However, this method fails to propose a definite approach for uniformly dispersing B4C throughout a C/C composite. In addition, oxidation resistance due to containing of B4C basically results from an oxidation-resistant protective coating of B2O3. At a temperature exceeding 800 degrees C., accordingly, B2O3 evaporates to provide no fundamental oxidation resistance, but nevertheless Japanese Patent No. 3058180 shows no evaluation for oxidation at a temperature around 800 degrees C.
Japanese Patent Laid-Open No. H04-214073 discloses an oxidation resistant C/C composite that comprises, as a matrix, carbon in which dispersed are 10 to 50 volume % of ceramic having a particle diameter of 10 μm or less (boron carbide, silicon carbide, etc.), and also comprises, as a reinforcing material, 20 to 40 volume % of long carbon fibers. However, most Examples therein exhibited weight loss in two to four hours even at 800 degrees C. in the atmosphere, and thereafter exhibited a tendency of further weight loss. Even the best one of these Examples exhibited weight loss in seven hours, and thereafter exhibited a tendency of further weight loss. Accordingly, the material disclosed therein substantially provides only a small availability. Like this, when considering a matter of costs, the existing circumstances see no oxidation resistant C/C composite available as industrial materials for general industry.
A C/C composite in the most common use today is a 2D-C/C composite using two-dimensional cloths. Therefore, as an oxidation-resistant material for general industry as well, the 2D-C/C composite is the most available and highly required. A production of an oxidation resistant C/C composite adopting the 2D-C/C composite is more difficult than a production of the same adopting a felt C/C composite or a 1D-C/C composite using one-dimensional carbon fibers. Thus, there has been an insufficient development for providing the whole of these materials with oxidation-resistance and self-sealing ability for practical use.
An object of the present invention is to provide an oxidation resistant C/C composite by means of a simple production process, which is widely available for general industry and provided with oxidation resistance under a high temperature in the atmosphere, without failing such characteristics of a C/C composite as high strength and high toughness.